Image forming apparatus having simple configuration and capable of measuring toner current included in developing current, and accurately calculating toner charge amount based on measurement result

Abstract

Provided is an image forming apparatus having a simple configuration capable of measuring a toner current included in a developing current and accurately calculating a toner charge amount based on the measurement result. A developing device has a developer carrier that carries a two-component developer including a magnetic carrier and toner. A developing voltage power supply applies a developing voltage obtained by superimposing an AC voltage on a DC voltage on the developer carrier. A control unit estimates a toner charge amount based on a toner current calculated by subtracting a carrier current from a developing current detected by a current detecting unit when a reference image is formed during non-image formation, and a toner developing amount calculated from the density of a reference image detected by a density detecting device.

What is claimed is: 
     
       1. An image forming apparatus, comprising:
 an image forming unit that comprises:
 an image carrier having a photosensitive layer formed on the surface thereof, 
 a charging device that charges the image carrier; 
 an exposing device that forms an electrostatic latent image by exposing the image carrier charged by the charging device; and 
 a developing device that has a developer carrier arranged facing the image carrier and carrying a two-component developer that includes a magnetic carrier and toner, and forms a toner image by adhering the toner to the electrostatic latent image formed on the image carrier; 
 
 a developing voltage power supply that applies a developing voltage obtained by superimposing an AC voltage on a DC voltage on the developer carrier; 
 a density detecting device that detects a density of the toner image formed by the developing device; 
 a current detecting unit that detects a DC component of a developing current that flows when the developing voltage is applied to the developer carrier; and 
 a control unit that controls the image forming unit and the developing voltage power supply; 
 the control unit, 
 by the developing device at a time of non-image formation, forms a reference image on the image carrier; by a current detecting unit, detects the developing current when the reference image is formed; calculates a toner current flowing due to movement of the toner by subtracting a carrier current flowing through the magnetic carrier from the detected developing current; 
 calculates a toner developing amount when the reference image is formed from a density of the reference image detected by the density detecting device; and 
 estimates the toner charge amount based on the toner current and the toner developing amount. 
 
     
     
       2. The image forming apparatus according to  claim 1 , wherein
 the control unit changes the developing potential difference V0−Vdc between a surface potential V0 of the image carrier and a DC component Vdc of the developing voltage applied to the developer carrier in a plurality of steps; detects a DC component of the developing current flowing through the developer carrier when facing a non-exposed portion of the image carrier as the carrier current; and using an approximation equation obtained from correlation between the V0−Vdc and the carrier current, estimates the carrier current that flows when the reference image is formed. 
 
     
     
       3. The image forming apparatus according to  claim 1 , wherein
 the control unit changes image formation conditions based on an estimated toner charge amount. 
 
     
     
       4. The image forming apparatus according to  claim 3 , wherein
 when the toner charge amount is lower than a specific value, 
 the control unit lowers a toner concentration in the developing apparatus, or lowers Vpp of an AC component of the developing voltage, or reduces a developing potential difference V0−Vdc between a surface potential V0 of the image carrier and a DC component Vdc of the developing voltage. 
 
     
     
       5. The image forming apparatus according to  claim 3 , further comprising
 a transfer member that is arranged facing the image carrier and that transfers the toner image formed on the image carrier to a subject to transfer; wherein 
 when the toner charge amount is lower than a specific value, 
 the control unit increases the transfer voltage applied to the transfer member. 
 
     
     
       6. The image forming apparatus according to  claim 1 , wherein
 when an estimated toner charge amount is outside of a specified range, 
 the control unit performs a forced ejection operation for ejecting the toner inside the developing device to the image carrier.